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This is a pre-fab panel house, which means the walls were delivered to a team on site that had built the foundation and floor. It's fascinating to learn how this approach impacts the final project (TLDR, it's a classic "throw it over the wall to the next team on the line" situation).

One issue we've discovered is at the connection between foundation and sill. The sill plate sits on tarpaper, on top of the foundation. The rim joist has a layer of wood fiber insulation on the outside. The siding (hardiboard) runs to approximately the top of the foundation, but there's a gap. The backfill is up to approximately the top of the foundation.

The gap between the siding, the rim joist, and the top of the foundation is a great place for critters to go in, chew that insulation, get in between furring strips and siding, and find holes into the crawlspace. I've seen wasps flying in and out. I've seen voles going in and out.

In an effort to protect the interior, I've foamed around the inside of the sill. And in a couple places I've foamed on the outside, just to plug a wasp nest. But most of the base of the walls on the outside is still exposed to the elements and creatures. Appx. 50% of it is under a deck.

I'm wondering if anyone has suggestions for making this better.

In the attached photo, you can see all of these elements: On the left, the siding comes down to the ground. On the right, the wood fiber insulation and furring strips. Under the wall, the tarpaper comes from the sill.

picture of construction

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    It's not so much the sill as the bottom edge of the rim insulation and the gaps between the furring strips that you're concerned about, right? The sill is just the first board on top of the foundation. Is your drain plane over or behind the furring strips?
    – isherwood
    Aug 3 '20 at 21:01
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An L or Z-shaped metal flashing is often used to bring water from behind a cladding out to the front of the wall (for example, when the upper part of a wall is clad with stucco/siding/etc and the lower part is clad with brick/stone/other masonry). Metal also makes a nice barrier against things that chew, burrow, and otherwise damage structures.

z flashing

You could clear away the soil from the top of the foundation (probably a good idea anyway to keep insects at bay) and insert this kind of flashing. Ideally it would slip up behind the fibrous insulation layer and would project forward to the outside face of the siding. In this way the metal blocks access to both the furring strip gap and also the insulation. Secure it in such a way that critters can't pass between the foundation concrete and the metal, nor between the metal and the bottom edge of the siding.

Avoid applying a sealant between the bottom of the siding and the metal. Up to now any moisture that gets into the wall can freely drain out the bottom, but sealing this flashing to the siding would trap liquid inside the wall.

Alternatively, use a flashing that is perforated along the bottom edge so that liquid can drain out. This kind of drainage flashing is sometimes used at the lower edge of EIFS stucco installations. The one I've used was extruded plastic, though, so it wouldn't keep out rodents with sharp teeth so well as metal could.

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  • This is a very helpful answer. Thank you. Do you think it would be reasonable to hope that a siding company (or some other contractor) could figure this out and do it, or is it just too unusual for that?
    – hoosteeno
    Aug 9 '20 at 22:46
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    A siding contractor should be familiar with and able to obtain off-the-shelf or make custom flashings. Every job is different (custom) at least a little bit. You'd have to describe your concern to them like you did here and in the ensuing conversation could tell them about the ideas that were explored here. A contractor worth hiring will listen to your concern, work out a solution, and carry it out. There's some balance though: even a good contractor might not want to discuss step-by-step details if he worries you're pumping him for free information so you can just do the job yourself.
    – Greg Hill
    Aug 10 '20 at 13:52
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I would fasten sheet metal angle (or Z-mold) flashing to the bottom of the furring strips before the siding goes on. The horizontal flange depth would be the 1-3/4" or 2-1/4" So that it's tight to the wooden sill or concrete foundation at the back, depending on height.

Like so (elevation view):

 |  |      |
 |  |      |
 | <|------|-- furring strip
||  |      |
||  |   <--|-- foam insulation
||  |      |
||  |      |
|__________|
    ^------|--- flashing
           |
           |<-- foundation or sill
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  • thanks for this and your comment above. You said, "Is your drain plane over or behind the furring strips?" and I don't know the answer. Is there a way to tell? All the siding is on. You're right that I mostly want to protect the cavity between hardiboard and wall system. Wall system has weatherproof layer on it, so I assume that's drainage plane. So maybe I could get a metal angle, jam it into the space where your "flashing" line is above? My worry there is that it'll angle down toward sill plate and drain backward.
    – hoosteeno
    Aug 9 '20 at 22:39
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    If your drain plane is at the sheathing this flashing doesn't change anything. Any water that gets behind the siding is already at the point where the flashing terminates.
    – isherwood
    Aug 10 '20 at 12:29

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